Polyamides derived from aliphatic diamine(s) and aliphatic or aromatic carboxylic diacid(s) have already been known for a long time. Depending on their composition, these polyamides are semi-crystalline polymers having high melting points (m.p.) or completely amorphous substances having relatively low glass transition temperatures (Tg).
Semi-crystalline polyamides are advantageously used when it is desired to produce formed articles which have a good thermomechanical stability at high temperature. The best known of the semi-crystalline polyamides is polyamide 6.6 (polymer of hexamethylenediamine and adipic acid); this polyamide is easily convertible by the melt route, but, because of a low Tg of about 50.degree. C., it finds fields of application which are restricted to those involving use temperatures which remain below 100.degree. C. Other semi-crystalline polyamides have been disclosed which have a higher Tg and, consequently, a better retention of the mechanical property values as a function of temperature. A polyamide of this type which is of particular value is that obtained from terephthalic. acid and 2-methylpentamethylene-1,5-diamine (cf. JP-A-69/019,551) because of the possibility of obtaining a Tg of the order of 142.degree. C.; the stability to heat is clearly noted with a polyamide of this the, but it has been found that:
if this polymer is filled with a padding or reinforcing filler of fibrous type and then converted by injection moulding under industrially economic conditions using a cold mould, that is to say a mould kept at a temperature of between 100.degree. and 150.degree. C., with short cooling times (the time concerned is the minimum time for which the injected material must be kept in the mould to enable the part to be ejected without deformation or to enable the moulded product to fall from the mould without difficulty), that is to say cooling times of between 20 and 40 seconds, PA1 it is not possible to obtain a high BTUL under 1.8 MPa able to reach and exceed 240.degree. C. It has been found that this BTUL is, for example, 145.degree.-150.degree. C. for moulded articles filled with 10 to 60% by weight of glass fibres (relative to the weight of the total polyamide+filler) and obtained by the cold mould technique. The production, under these moulding conditions, of a low value for the BTUL is to be related to a low capacity for crystallisation of the polyamide under consideration obtained from terephthalic acid and alkylpentamethylenediamine, which has the result that when cooled rapidly, in particular when it is injected into a cold mould, the said polyamide does not crystallise or does so only partially. The articles obtained may then be amorphous; moreover, they are liable to change as a result of a crystallisation during reheating. It is understood that this type of semi-aromatic polyamide is useable only when using a hot mould, that is to say a mould kept at a temperature of between 170.degree. and 200.degree. C., with long cooling times, that is to say cooling times of between 2 and 4 minutes, and consequently their development remains very limited. PA1 that the rate of crystallisation may be increased to a level such that it is possible to produce mouldings in a cold mould and to obtain, in particular, moulded articles having, when they are filled with at least one padding or reinforcing filler of fibrous typer a FTUL under 1.8 MPa of at least 240.degree. C., PA1 by preparing copolyamides by using, alongside terephthalic acid, an amine reagent comprising an alkylpentamethylenediamine and hexamethylenediamine. PA1 the molecular ratio of units (I) with respect to the sum of units (II)+(III)+(IV) is 1; PA1 the amount of units (III) in the mixture (II)+(III) is in the range ranging from 0 to 5 mol % and that of the units (II), with respect to the same reference, is in the range ranging from 100 to 95 mol %; PA1 the amount of units (IV) in the mixture (II)+(III)+(IV) is in the range ranging from 40 to 90 mol %; and PA1 they also contain a proportion of non-conforming units derived from bis-hexamethylenetriamine, expressed as a percentage by weight of bis-hexamethylenetriamine with respect to the weight of the copolyamide obtained, which is less than 0.5% and more precisely is within the range ranging from 0.01 to 0.5%. PA1 viscosity characteristics, expressed as the viscosity index (IV) measured in accordance with the standard ISO R 307 (2nd Edition, 1984) on a solution containing 0.5 g of polymer (dried for 2 hours. at 60.degree. C. while sweeping with argon) in 100 cm.sup.a of metacresol or of 95% by weight concentrated H.sub.2 SO.sub.4, which are at least 60 ml/g and may be between 80 and 250 ml/g; and PA1 number-average molecular masses (Mn), determined by determining the end groups and the non-conforming units, which are at least 9,000 g/mol and maybe between 10,000 and 25,000 g/mol. PA1 Tgs, measured by DSC analysis, which are at least 120.degree. C. and may be between 125.degree. and 140.degree. C.; and PA1 BTULs under 1.8 MPa, determined in accordance with the standard NF T 51005, which are at least 240.degree. C. and may be between 250.degree. and 310.degree. C., when the copolyamides are filled with, for example, 10 to 60% by weight (with respect to the weight of copolyamide+filler) of at least one padding or reinforcing filler of fibrous nature. PA1 Step 1: in which, the autoclave being closed, the temperature of the starting composition is progressively increased up to a specific value T1 which is within the range ranging from 170.degree. C. to 240.degree. C.; then, under a constant pressure P equal to the autogenous water vapour pressure obtained, which is within the range ranging from 0.7 to 2.6 MPa, the water present in the reaction mass is removed by regular distillation, whilst progressively raising the temperature of the composition at the same time to a value T2 which is higher than the temperature T1 reached before distillation and is within the range ranging from215.degree. C. to 320.degree. C.; PA1 Step 2: in which the pressure is progressively lowered from the autogenous pressure value to atmospheric pressure value and, optionally, the same time, the temperature of the composition is raised to a value T3 which is higher by ten to several tens of degrees celsius than the temperature T2 reached before decompression, whilst continuing to ensure regular distillation of water throughout this decompression period; PA1 Step 3: in which the polycondensation reaction is completed by stirring the reaction composition for a certain t{me, carrying out the reaction under atmospheric pressure and optionally/or under a lower pressure with a mass temperature equal to or higher than the temperature T2 or T3 obtained at the end of Step 2, for a sufficient period enabling a polyamide having the desired molecular and viscosity characteristics to be obtained. PA1 Step 2': in which the reactor is emptied rapidly, over a period ranging from 5 minutes to 30 minutes: during this period, the autogenous water vapour pressure is lowered from the autogenous pressure value to atmospheric pressure value and, working under an inert atmosphere (nitrogen), the prepolymer formed is recovered and then cooled and converted to a powder by grinding; PA1 Step 3': in which the reaction for completion of the polycondensation is carried out by conducting the post-condensation of the prepolymer in powder form obtained: either in the solid phase, working in a reactor kept under a pressure, optionally in the presence of inert gas (nitrogen), of between the atmospheric pressure value and a lower value of 0.1.10.sup.2 Pa, at a temperature ranging from 200.degree. C. to 280.degree. C. and for a sufficient period enabling a copolyamide to be obtained, having the desired molecular and viscosity characteristics, ranging, for example, from 10 minutes to 5 hours; or in the molten phase, working in an extruder-degasser containing one or more screws, the essential operating parameters of the said extruder-degasser comprisings the temperature of the reaction zone, which is within the range ranging from 310.degree. C. to 360.degree. C., the devolatilisation pressure for this zone, which is within the range ranging from the atmospheric pressure value to a lower value of 0.1.0.sup.2 Pa, and the residence time of the prepolymer in the extruder-degasset, which is within the range ranging from 30 seconds to 5 minutes; or by linking post-condensation in the molten phase and post-condensation in the solid phase. PA1 Step 1": conducted under an inert atmosphere (nitrogen), in which step the starting composition, containing the elements (i) and (3i), which have been referred to above with regard to the starting mixture for the first and second processes, is dissolved in an aprotic polar solvent or a mixture of such solvents having a boiling point higher than 220.degree. C., working at ambient temperature of 20.degree. C. to 30.degree. C., the temperature of the solution obtained is then raised to the desired reaction temperature of between 190.degree. C. and 220.degree. C. and the water of amidification is removed by regular distillation under atmospheric pressure for a period ranging from 20 minutes to 2 hours; PA1 Step 3": in which, once the solvent or solvents have been removed, the polycondensation is completed by allowing the reaction mass to react at a temperature ranging from 300.degree. to 340.degree. C., operating under a reduced pressure which is within the range ranging from 0.1.10.sup.2 Pa to 1.10.sup.2 Pa for a sufficient period (including the period for applying reduced pressure) enabling a copolyamide to be obtained having the desired molecular and viscosity characteristics, ranging, for example, from 10 minutes to 1 hour. PA1 on the one hand, a loss of total basicity involving, on the one hand, a loss of stoichiometry during the polycondensation reaction between the primary amino groups and the carboxyl groups which are reacting, consequently precluding the possibility of easily increasing the molecular mass of the polyamide being formed, and, on the other hand, a real difficulty for industrial reproduction of the process employed. The loss in total basicity which has been mentioned above is established with respect to the total amount of amine reagent employed and is expressed by the equation. ##EQU2## the expression "basicity lost" corresponds to the sum: number of NH.sub.2 equivalents of the amine reagent lost during distillation+number of NH equivalents of free cyclic amine+number of NH.sub.2 equivalents of ammonia. This lost basicity is determined directly, by means of a potentiometric determination, on the distillates, that is to say all of the water condensed during the distillation stages under constant pressure and during decompression; PA1 the expression "basicity used" corresponds to the number of NH.sub.2 equivalents of the amine reagent used. The expression "number of equivalents" of primary or secondary amino of a compound defines the number of primary or secondary amino groups contained in one mol of the said compound; for example one mol of amine reagent comprising 2-methylpentamethylenediamine contains 2 primary amino equivalents NH.sub.2 whereas one mol of cyclic amine comprising 3-methylpiperidine contains one secondary amino equivalent NH: PA1 and, on the other hand, the existence in the polycondensation mixture of a high proportion of terminal groups of the cyclic amine type, which have the role of limiting the chain and may limit access to high molecular masses. PA1 the polymer first undergoes hydrolysis in hydrochloric acid and is then neutralised with sodium hydroxide solution to a slightly basic pH, PA1 the amines are then extracted quantitatively with chloroform, and PA1 this chloroform extract is finally analysed by gas phase chromatography and the 3-methylpiperidine is determined by the internal standard methods. The number of terminal MPP groups (MPP TG) in the polymer is in this case also expressed in meq/kg. PA1 the polymer first undergoes hydrolysis in hydrochloric acid and is then neutralised with sodium hydroxide solution to a slightly basic pH, PA1 the amines are then extracted quantitatively with chloroform and the chloroform extract obtained is concentrated by evaporation of chloroform, and then PA1 the concentrated solution obtained is finally analysed by gas phase chromatography and the BHT is determined by the internal standard method. The BHT content will be given in millimoles per kilogram of polymer (mM/kg) and in % by weight in the polymer (1 mM of BHT=0.215 g). PA1 lowering from 350.degree. C. to 25.degree. C. at a rate of 10.degree. C./min; this step serves to obtain the crystallisation temperature on cooling (Tcr); PA1 reheating from 25.degree. C. to 350.degree. C. at a rate of 10.degree. C./min; this step serves to obtain the glass transition temperature (Tg) and the melting point (m.p.) of the polymer.
European Patent No. 0,347,848 also discloses semi-crystalline polyamides obtained from terephthalic acid, 2-methylpentamethylenediamine and an aromatic amine.
The use of a semi-crystalline, semi-aromatic polyamide based on recurring units derived from terephthalic acid and alkYlpentamethylenediamine, for moulding under industrially economical conditions, therefore rests on the need to modify its capacity for crystallisation. On leaving the cold mould, the polymer must have acquired a semi-crystalline morphology throughout the mass of the part.